Justin Barrasso has an article at Sports Illustrated with WWE Hall of Famer Bret "The Hitman" Hart talking about the amount of respect The Undertaker had backstage. Below are a couple of highlights:

Bret Hart writing The Undertaker a letter when he left WWF in 1997:

"I wrote 'Taker a letter when I left. I said, 'You have to be the voice of the dressing room and the leader now that I'm gone.' I always felt that I was the leader of the dressing room and looked after everything. If guys had a problem, they could come to me and I'd go to Vince and say, 'This guy has a problem, maybe you can help him.' Then 'Taker stepped in and became that guy. A lot of times he had to straighten out guys like Shawn Michaels and stand up for the other wrestlers. Undertaker was always a force in the dressing room."